Doubly fed generators can output at their stator an AC voltage having the same frequency as that in a grid voltage frequency by exciting a rotor winding with a slip frequency by a converter and provide relative freedom of the rotational frequency and reduction in capacity of a converter. Japanese laid-open patent application publication No. 2000-308398 discloses such a doubly fed generator. Further, due to the relative freedom of the rotational frequency in generation operation and the reduction in capacity of the converter, the doubly fed generator is used for wind turbine generators. U.S. Pat. Ser. No. 6,566,764 discloses such a wind turbine generator system.
In the conventional wind turbine generator system employing a doubly fed generator, a converter has a lower capacity than the generator frequently. Thus, when the rotation frequency reaches a predetermined range, the wind turbine generator system should frequently repeat starts and stops due to affection of variation in wind power because the generator should be operated in parallel to, synchronously with the grid. This elongates a time interval necessary for synchronous operation with the grid. Thus, there may be intervals in which power cannot be generated though wind is blowing. This decreases availability of the wind turbine generator system.